The ink jet head is applied to an ink jet recording method (liquid discharge recording method) in which the recording is performed by discharging a recording solution such as ink. The ink jet head generally includes an ink flow path, a liquid discharge energy generating portion provided in a part of the ink flow path, and a fine ink discharge port (also referred to as “orifice”) for discharging the ink in the ink flow path by energy of the liquid discharge energy generating portion. With reference to the conventional method of producing the ink jet head, for example, Japanese Patent Publication No. HO 6-045242 discloses a method for manufacturing an ink jet head (also referred to as cast molding method) in which a mold of the ink flow path is patterned onto the substrate, in which liquid discharge energy generating elements are formed, by a photosensitive material, a coating resin layer is applied onto the substrate so that the mold pattern is coated, an ink discharge port communicated with the mold of the ink flow path is formed in the coated resin layer, and then the photosensitive material used for the mold is removed. From the viewpoint of easy removal, a positive type resist is used as the photosensitive material in the method for manufacturing an ink jet head. Further, according to the method for manufacturing an ink jet head, because a technique of semiconductor lithography is applied, fine processing can be realized with extremely high accuracy for the formation of the ink flow path, the ink discharge port, and the like.
However, since a negative type resist is applied onto the ink flow path pattern formed by the positive type resist, sometimes there is generated a problem that the ink flow path pattern is dissolved and deformed during the application of the negative type resist.
In order to avoid the problem in the conventional ink flow path patterning, for example, Japanese Patent Application Laid-Open No. H08-323985 discloses a method in which the negative type resist is applied after solvent-resistance properties are improved by performing intermolecular crosslinking with an ionizing radiation decomposition type photosensitive resin composition including an intermolecular crosslinkable structural unit. It is the method of performing the intermolecular crosslinking by baking the photosensitive resin containing an 8/2 copolymer (weight average molecular weight is 180000) of methyl methacrylate/methacrylic acid at 180° C. for one hour.
Further, in Japanese Patent Application Laid-Open No. 2004-042396, the inventors propose that an acrylic copolymer containing methacrylate ester as a main component as the further preferable acrylic resin, containing methacrylic acid as a thermal crosslinking factor at a proportion of 2 to 30 weight %, and whose molecular weight ranges 5000 to 50000, is used by performing thermal crosslinking of acrylic copolymer for the,positive type resist for forming the ink flow path.
According to these methods, although the deformation of the ink flow path pattern can be prevented, the following problems still exist:    (1) Due to the intermolecular crosslinking, a large amount of energy is required for a photodegradation reaction of the positive type resist, and sensitivity tends to decrease. Further, because progress of the photodegradation reaction is insufficient, particularly when the positive type resist is used in a thick film, sometimes a decrease in resolution is generated.    (2) When the positive type resist is used in the thick film, sometimes a crack is generated by curing shrinkage stress associated with the intermolecular crosslinking. Further, sometimes the crack is generated in development or in the application of the negative type resist.    (3) In order to impart the sufficient solvent-resistance properties, heat treatment is required at high temperatures for a long time.
Therefore, a width or a height of the ink flow path is restricted, which results in not only an obstacle of ink flow path design but also a decrease in production.